BESS Workgroup J. Rabadan, Ed.
Internet Draft Nokia
S. Mohanty, Ed.
Intended status: Standards Track A. Sajassi
Cisco
J. Drake
Juniper
K. Nagaraj
S. Sathappan
Nokia
Expires: April 22, 2019 October 19, 2018
Framework for EVPN Designated Forwarder Election Extensibilitydraft-ietf-bess-evpn-df-election-framework-04
Abstract
The Designated Forwarder (DF) in EVPN networks is the Provider Edge
(PE) router responsible for sending broadcast, unknown unicast and
multicast (BUM) traffic to a multi-homed Customer Equipment (CE)
device, on a given VLAN on a particular Ethernet Segment (ES). The DF
is selected out of a list of candidate PEs that advertise the same
Ethernet Segment Identifier (ESI) to the EVPN network. By default,
EVPN uses a DF Election algorithm referred to as "Service Carving"
and it is based on a modulus function (V mod N) that takes the number
of PEs in the ES (N) and the VLAN value (V) as input. This default DF
Election algorithm has some inefficiencies that this document
addresses by defining a new DF Election algorithm and a capability to
influence the DF Election result for a VLAN, depending on the state
of the associated Attachment Circuit (AC). In addition, this document
creates a registry with IANA, for future DF Election Algorithms and
Capabilities. It also presents a formal definition and clarification
of the DF Election Finite State Machine.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that
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This Internet-Draft will expire on April 22, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Auto-Discovery per Ethernet Segment route.
o Ethernet A-D per EVI route - refers to [RFC7432] route type 1 or
Auto-Discovery per EVPN Instance route.
o ES and ESI - Ethernet Segment and Ethernet Segment Identifier.
o EVI - EVPN Instance.
o BD - Broadcast Domain. An EVI may be comprised of one (VLAN-Based
or VLAN-Bundle services) or multiple (VLAN-Aware Bundle services)
Broadcast Domains.
o HRW - Highest Random Weight
o VID and CE-VID - VLAN Identifier and Customer Equipment VLAN
Identifier.
o Ethernet Tag - used to represent a Broadcast Domain that is
configured on a given ES for the purpose of DF election. Note that
any of the following may be used to represent a Broadcast Domain:
VIDs (including double Q-in-Q tags), configured IDs, VNI,
normalized VID, I-SIDs, etc., as long as the representation of the
broadcast domains is configured consistently across the multi-homed
PEs attached to that ES. The Ethernet Tag value MUST be different
from zero.
o Ethernet Tag ID - refers to the identifier used in the EVPN routes
defined in [RFC7432]. Its value may be the same as the Ethernet Tag
value (see Ethernet Tag definition) when advertising routes for
VLAN-aware bundle services. Note that in case of VLAN-based or VLAN
Bundle services, the Ethernet Tag ID is zero.
o DF Election Procedure and DF Algorithm - The Designated Forwarder
Election Procedure or simply DF Election, refers to the process in
its entirety, including the discovery of the PEs in the ES, the
creation and maintenance of the PE candidate list and the selection
of a PE. The Designated Forwarder Algorithm is just a component of
the DF Election Procedure and strictly refers to the selection of a
PE for a given <ES,Ethernet Tag>.
This document also assumes familiarity with the terminology of
[RFC7432].
2. Introduction2.1. Default Designated Forwarder (DF) Election in EVPNRabadan, Mohanty et al. Expires April 22, 2019 [Page 4]

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[RFC7432] defines the Designated Forwarder (DF) as the EVPN PE
responsible for:
o Flooding Broadcast, Unknown unicast and Multicast traffic (BUM), on
a given Ethernet Tag on a particular Ethernet Segment (ES), to the
CE. This is valid for single-active and all-active EVPN
multi-homing.
o Sending unicast traffic on a given Ethernet Tag on a particular ES
to the CE. This is valid for single-active multi-homing.
Figure 1 illustrates an example that we will use to explain the
Designated Forwarder function.
+---------------+
| IP/MPLS |
| CORE |
+----+ ES1 +----+ +----+
| CE1|-----| |-----------| |____ES2
+----+ | PE1| | PE2| \
| |-------- +----+ \+----+
+----+ | | | CE2|
| | +----+ /+----+
| |__| |____/ |
| | PE3| ES2 /
| +----+ /
| | /
+-------------+----+ /
| PE4|____/ES2
| |
+----+
Figure 1 Multi-homing Network of EVPN
Figure 1 illustrates a case where there are two Ethernet Segments,
ES1 and ES2. PE1 is attached to CE1 via Ethernet Segment ES1 whereas
PE2, PE3 and PE4 are attached to CE2 via ES2 i.e. PE2, PE3 and PE4
form a redundancy group. Since CE2 is multi-homed to different PEs on
the same Ethernet Segment, it is necessary for PE2, PE3 and PE4 to
agree on a DF to satisfy the above mentioned requirements.
Layer-2 devices are particularly susceptible to forwarding loops
because of the broadcast nature of the Ethernet traffic. Therefore it
is very important that, in case of multi-homing, only one of the
links be used to direct traffic to/from the core.
One of the pre-requisites for this support is that participating PEs
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must agree amongst themselves as to who would act as the Designated
Forwarder (DF). This needs to be achieved through a distributed
algorithm in which each participating PE independently and
unambiguously selects one of the participating PEs as the DF, and the
result should be consistent and unanimous.
The default algorithm for DF election defined by [RFC7432] at the
granularity of (ESI,EVI) is referred to as "service carving". In this
document, service carving or default DF Election algorithm are used
interchangeably. With service carving, it is possible to elect
multiple DFs per Ethernet Segment (one per EVI) in order to perform
load-balancing of traffic destined to a given Segment. The objective
is that the load-balancing procedures should carve up the BD space
among the redundant PE nodes evenly, in such a way that every PE is
the DF for a distinct set of EVIs.
The DF Election algorithm as described in [RFC7432] (Section 8.5) is
based on a modulus operation. The PEs to which the ES (for which DF
election is to be carried out per EVI) is multi-homed form an ordered
(ordinal) list in ascending order of the PE IP address values. For
example, there are N PEs: PE0, PE1,... PEN-1 ranked as per increasing
IP addresses in the ordinal list; then for each VLAN with Ethernet
Tag V, configured on the Ethernet Segment ES1, PEx is the DF for VLAN
V on ES1 when x equals (V mod N). In the case of VLAN-Bundle only the
lowest VLAN is used. In the case when the planned density is high
(meaning there are significant number of VLANs and the Ethernet Tags
are uniformly distributed), the thinking is that the DF Election will
be spread across the PEs hosting that Ethernet Segment and good load-
balancing can be achieved.
However, the described default DF Election algorithm has some
undesirable properties and in some cases can be somewhat disruptive
and unfair. This document describes some of those issues and proposes
a mechanism for dealing with them. These mechanisms do involve
changes to the default DF Election algorithm, but they do not require
any changes to the EVPN Route exchange and have minimal changes to
their content per se.
In addition, there is a need to extend the DF Election procedures so
that new algorithms and capabilities are possible. A single algorithm
(the default DF Election algorithm) may not meet the requirements in
all the use-cases.
Note that while [RFC7432] elects a DF per <ES, EVI>, this document
elects a DF per <ES, BD>. This means that unlike [RFC7432], where for
a VLAN Aware Bundle service EVI there is only one DF for the EVI,
this document specifies that there will be multiple DFs, one for each
BD configured in that EVI.
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Internet-Draft DF Election Framework for EVPN October 19, 20182.2. Problem Statement
This section describes some potential issues with the default DF
Election algorithm.
2.2.1. Unfair Load-Balancing and Service Disruption
There are three fundamental problems with the current default DF
Election algorithm.
1- First, the algorithm will not perform well when the Ethernet Tag
follows a non-uniform distribution, for instance when the Ethernet
Tags are all even or all odd. In such a case let us assume that
the ES is multi-homed to two PEs; one of the PEs will be elected
as DF for all of the VLANs. This is very sub-optimal. It defeats
the purpose of service carving as the DFs are not really evenly
spread across. In fact, in this particular case, one of the PEs
does not get elected as DF at all, so it does not participate in
the DF responsibilities at all. Consider another example where,
referring to Figure 1, lets assume that PE2, PE3, PE4 are in
ascending order of the IP address; and each VLAN configured on ES2
is associated with an Ethernet Tag of the form (3x+1), where x is
an integer. This will result in PE3 always be selected as the DF.
2- Even in the case when the Ethernet Tag distribution is uniform the
instance of a PE being up or down results in re-computation ((v
mod N-1) or (v mod N+1) as is the case); the resulting modulus
value need not be uniformly distributed because it can be subject
to the primality of N-1 or N+1 as may be the case.
3- The third problem is one of disruption. Consider a case when the
same Ethernet Segment is multi homed to a set of PEs. When the ES
is down in one of the PEs, say PE1, or PE1 itself reboots, or the
BGP process goes down or the connectivity between PE1 and an RR
goes down, the effective number of PEs in the system now becomes
N-1, and DFs are computed for all the VLANs that are configured on
that Ethernet Segment. In general, if the DF for a VLAN v happens
not to be PE1, but some other PE, say PE2, it is likely that some
other PE will become the new DF. This is not desirable. Similarly
when a new PE hosts the same Ethernet Segment, the mapping again
changes because of the modulus operation. This results in needless
churn. Again referring to Figure 1, say v1, v2 and v3 are VLANs
configured on ES2 with associated Ethernet Tags of value 999, 1000
and 1001 respectively. So PE1, PE2 and PE3 are the DFs for v1, v2
and v3 respectively. Now when PE3 goes down, PE2 will become the
DF for v1 and PE1 will become the DF for v2.
One point to note is that the default DF election algorithm assumes
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that all the PEs who are multi-homed to the same Ethernet Segment
(and interested in the DF Election by exchanging EVPN routes) use an
Originating Router's IP Address of the same family. This does not
need to be the case as the EVPN address-family can be carried over a
v4 or v6 peering, and the PEs attached to the same ES may use an
address of either family.
Mathematically, a conventional hash function maps a key k to a number
i representing one of m hash buckets through a function h(k) i.e.
i=h(k). In the EVPN case, h is simply a modulo-m hash function viz.
h(v) = v mod N, where N is the number of PEs that are multi-homed to
the Ethernet Segment in discussion. It is well-known that for good
hash distribution using the modulus operation, the modulus N should
be a prime-number not too close to a power of 2 [CLRS2009]. When the
effective number of PEs changes from N to N-1 (or vice versa); all
the objects (VLAN V) will be remapped except those for which V mod N
and V mod (N-1) refer to the same PE in the previous and subsequent
ordinal rankings respectively. From a forwarding perspective, this is
a churn, as it results in re-programming the PE ports as either
blocking or non-blocking at potentially all PEs when the DF changes.
This document addresses this problem and furnishes a solution to this
undesirable behavior.
2.2.2. Traffic Black-Holing on Individual AC Failures
As discussed in section 2.1 the default DF Election algorithm defined
by [RFC7432] takes into account only two variables in the modulus
function for a given ES: the existence of the PE's IP address on the
candidate list and the locally provisioned Ethernet Tags.
If the DF for an <ESI, EVI> fails (due to physical link/node
failures) an ES route withdrawal will make the Non-DF (NDF) PEs re-
elect the DF for that <ESI, EVI> and the service will be recovered.
However, the default DF election procedure does not provide a
protection against "logical" failures or human errors that may occur
at service level on the DF, while the list of active PEs for a given
ES does not change. These failures may have an impact not only on the
local PE where the issue happens, but also on the rest of the PEs of
the ES. Some examples of such logical failures are listed below:
a) A given individual Attachment Circuit (AC) defined in an ES is
accidentally shutdown or even not provisioned yet (hence the
Attachment Circuit Status - ACS - is DOWN), while the ES is
operationally active (since the ES route is active).
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b) A given MAC-VRF - with a defined ES - is shutdown or not
provisioned yet, while the ES is operationally active (since the
ES route is active). In this case, the ACS of all the ACs defined
in that MAC-VRF is considered to be DOWN.
Neither (a) nor (b) will trigger the DF re-election on the remote
multi-homed PEs for a given ES since the ACS is not taken into
account in the DF election procedures. While the ACS is used as a DF
election tie-breaker and trigger in VPLS multi-homing procedures
[VPLS-MH], there is no procedure defined in EVPN [RFC7432] to trigger
the DF re-election based on the ACS change on the DF.
Figure 2 illustrates the described issue with an example.
+---+
|CE4|
+---+
|
PE4 |
+-----+-----+
+---------------| +-----+ |---------------+
| | | BD-1| | |
| +-----------+ |
| |
| EVPN |
| |
| PE1 PE2 PE3 |
| (NDF) (DF) (NDF)|
+-----------+ +-----------+ +-----------+
| | BD-1| | | | BD-1| | | | BD-1| |
| +-----+ |-------| +-----+ |-------| +-----+ |
+-----------+ +-----------+ +-----------+
AC1\ ES12 /AC2 AC3\ ES23 /AC4
\ / \ /
\ / \ /
+----+ +----+
|CE12| |CE23|
+----+ +----+
Figure 2 Default DF Election and Traffic Black-Holing
BD-1 is defined in PE1, PE2, PE3 and PE4. CE12 is a multi-homed CE
connected to ES12 in PE1 and PE2. Similarly CE23 is multi-homed to
PE2 and PE3 using ES23. Both, CE12 and CE23, are connected to BD-1
through VLAN-based service interfaces: CE12-VID 1 (VLAN ID 1 on CE12)
is associated to AC1 and AC2 in BD-1, whereas CE23-VID 1 is
associated to AC3 and AC4 in BD-1. Assume that, although not
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represented, there are other ACs defined on these ES mapped to
different BDs.
After running the [RFC7432] default DF election algorithm, PE2 turns
out to be the DF for ES12 and ES23 in BD-1. The following issues may
arise:
a) If AC2 is accidentally shutdown or even not configured, CE12
traffic will be impacted. In case of all-active multi-homing, the
BUM traffic to CE12 will be "black-holed", whereas for single-
active multi-homing, all the traffic to/from CE12 will be
discarded. This is due to the fact that a logical failure in PE2's
AC2 may not trigger an ES route withdrawn for ES12 (since there
are still other ACs active on ES12) and therefore PE1 will not re-
run the DF election procedures.
b) If the Bridge Table for BD-1 is administratively shutdown or even
not configured yet on PE2, CE12 and CE23 will both be impacted:
BUM traffic to both CEs will be discarded in case of all-active
multi-homing and all traffic will be discarded to/from the CEs in
case of single-active multi-homing. This is due to the fact that
PE1 and PE3 will not re-run the DF election procedures and will
keep assuming PE2 is the DF.
Quoting [RFC7432], "when an Ethernet Tag is decommissioned on an
Ethernet Segment, then the PE MUST withdraw the Ethernet A-D per EVI
route(s) announced for the <ESI, Ethernet Tags> that are impacted by
the decommissioning", however, while this A-D per EVI route
withdrawal is used at the remote PEs performing aliasing or backup
procedures, it is not used to influence the DF election for the
affected EVIs.
This document adds an optional modification of the DF Election
procedure so that the ACS may be taken into account as a variable in
the DF election, and therefore EVPN can provide protection against
logical failures.
2.3. The Need for Extending the Default DF Election in EVPNSection 2.2 describes some of the issues that exist in the default DF
Election procedures. In order to address those issues, this document
introduces a new DF Election framework. This framework allows the PEs
to agree on a common DF election algorithm, as well as the
capabilities to enable during the DF Election procedure Generally,
'DF election algorithm' refers to the algorithm by which a number of
input parameters are used to determine the DF PE, while 'DF election
capability' refers to an additional feature that can be used prior to
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the invocation of the DF election algorithm, such as modifying the
inputs (or list of candidate PEs).
Within this framework, this document defines a new DF Election
algorithm and a new capability that can influence the DF Election
result:
o The new DF Election algorithm is referred to as "Highest Random
Weight" (HRW). The HRW procedures are described in section 4.
o The new DF Election capability is referred to as "AC-Influenced DF
Election" (AC-DF). The AC-DF procedures are described in section 5.
o HRW and AC-DF mechanisms are independent of each other. Therefore,
a PE MAY support either HRW or AC-DF independently or MAY support
both of them together. A PE MAY also support AC-DF capability along
with the default DF election algorithm per [RFC7432].
In addition, this document defines a way to indicate the support of
HRW and/or AC-DF along with the EVPN ES routes advertised for a given
ES. Refer to section 3.2 for more details.
3. Designated Forwarder Election Protocol and BGP Extensions
This section describes the BGP extensions required to support the new
DF Election procedures. In addition, since the specification in EVPN
[RFC7432] does leave several questions open as to the precise final
state machine behavior of the DF election, section 3.1 describes
precisely the intended behavior.
3.1 The DF Election Finite State Machine (FSM)
Per [RFC7432], the FSM described in Figure 3 is executed per
<ESI,VLAN> in case of VLAN-based service or <ESI,[VLANs in VLAN-
Bundle]> in case of VLAN-Bundle on each participating PE.
Observe that currently the VLANs are derived from local configuration
and the FSM does not provide any protection against misconfiguration
where the same (EVI,ESI) combination has different set of VLANs on
different participating PEs or one of the PEs elects to consider
VLANs as VLAN-Bundle and another as separate VLANs for election
purposes (service type mismatch).
The FSM is conceptual and any design or implementation MUST comply
with a behavior equivalent to the one outlined in this FSM.
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4. DF_TIMER: DF Wait timer has expired.
5. RCVD_ES: A new or changed Ethernet Segment Route is received in a
BGP REACH UPDATE. Receiving an unchanged UPDATE MUST NOT trigger
this event.
6. LOST_ES: A BGP UNREACH UPDATE for a previously received Ethernet
Segment route has been received. If an UNREACH is seen for a
route that has not been advertised previously, the event MUST NOT
be triggered.
7. CALCULATED: DF has been successfully calculated.
According actions when transitions are performed or states
entered/exited:
1. ANY STATE on ES_DOWN: (i) stop DF timer (ii) assume non-DF for
local PE.
2. INIT on ES_UP: transition to DF_WAIT.
3. INIT on VLAN_CHANGE, RCVD_ES, LOST_ES: do nothing.
4. DF_WAIT on entering the state: (i) start DF timer if not started
already or expired (ii) assume non-DF for local PE.
5. DF_WAIT on VLAN_CHANGE, RCVD_ES, LOST_ES: do nothing.
6. DF_WAIT on DF_TIMER: transition to DF_CALC.
7. DF_CALC on entering or re-entering the state: (i) rebuild
candidate list, hash and perform election (ii) Afterwards FSM
generates CALCULATED event against itself.
8. DF_CALC on VLAN_CHANGE, RCVD_ES, LOST_ES: do nothing.
9. DF_CALC on CALCULATED: mark election result for VLAN or bundle,
and transition to DF_DONE.
11. DF_DONE on exiting the state: if there is a new DF election
triggered and the current DF is lost, then assume non-DF for
local PE for VLAN or VLAN-Bundle.
12. DF_DONE on VLAN_CHANGE, RCVD_ES or LOST_ES: transition to
DF_CALC.
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Internet-Draft DF Election Framework for EVPN October 19, 20183.2 The DF Election Extended Community
For the DF election procedures to be consistent and unanimous, it is
necessary that all the participating PEs agree on the DF Election
algorithm and capabilities to be used. For instance, it is not
possible that some PEs continue to use the default DF Election
algorithm and some PEs use HRW. For brown-field deployments and for
interoperability with legacy PEs, it is important that all PEs need
to have the capability to fall back on the Default DF Election. A PE
can indicate its willingness to support HRW and/or AC-DF by signaling
a DF Election Extended Community along with the Ethernet Segment
Route (Type-4).
The DF Election Extended Community is a new BGP transitive extended
community attribute [RFC4360] that is defined to identify the DF
election procedure to be used for the Ethernet Segment. Figure 4
shows the encoding of the DF Election Extended Community.
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0x06 | Sub-Type(0x06)| DF Alg | Bitmap |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitmap | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 DF Election Extended Community
Where:
o Type is 0x06 as registered with IANA for EVPN Extended Communities.
o Sub-Type is 0x06 - "DF Election Extended Community" as requested by
this document to IANA.
o DF Alg (1 octet) - Encodes the DF Election algorithm values
(between 0 and 255) that the advertising PE desires to use for the
ES. This document requests IANA to set up a registry called "DF Alg
Registry" and solicits the following values:
- Type 0: Default DF Election algorithm, or modulus-based algorithm
as in [RFC7432].
- Type 1: HRW algorithm (explained in this document).
- Types 2-254: Unassigned.
- Type 255: Reserved for Experimental Use.
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o Bitmap (2 octets) - Encodes "capabilities" to use with the DF
Election algorithm in the field "DF Alg". This document requests
IANA to create a registry for the Bitmap field, with values 0-15,
called "DF Election Capabilities" and solicits the following
values:
- Bit 0: Unassigned.
- Bit 1: AC-DF (AC-Influenced DF Election, explained in this
document). When set to 1, it indicates the desire to use AC-
Influenced DF Election with the rest of the PEs in the ES.
- Bits 2-15: Unassigned.
The DF Election Extended Community is used as follows:
o A PE SHOULD attach the DF Election Extended Community to any
advertised ES route and the Extended Community MUST be sent if the
ES is locally configured with a DF election algorithm other than
the Default Election algorithm or if a capability is required to be
used. In the Extended Community, the PE indicates the desired "DF
Alg" algorithm and "Bitmap" capabilities to be used for the ES.
- Only one DF Election Extended Community can be sent along with an
ES route. Note that the intent is not for the advertising PE to
indicate all the supported DF election algorithms and
capabilities, but signal the preferred one.
- DF Algs 0 and 1 can be both used with bit AC-DF set to 0 or 1.
- In general, a specific DF Alg MAY determine the use of the
reserved bits in the Extended Community, which may be used in a
different way for a different DF Alg.
o When a PE receives the ES Routes from all the other PEs for the ES
in question, it checks to see if all the advertisements have the
extended community with the same DF Alg and Bitmap:
- In the case that they do, this particular PE MUST follow the
procedures for the advertised DF Alg and capabilities. For
instance, if all ES routes for a given ES indicate DF Alg HRW and
AC-DF set to 1, the receiving PE and by induction all the other
PEs in the ES will proceed to do DF Election as per the HRW
Algorithm and following the AC-DF procedures.
- Otherwise if even a single advertisement for the type-4 route is
not received with the locally configured DF Alg and capability,
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the default DF Election algorithm (modulus) algorithm MUST be
used as in [RFC7432].
- The absence of the DF Election Extended Community MUST be
interpreted by a receiving PE as an indication of the default DF
Election algorithm on the sending PE, that is, DF Alg 0 and no DF
Election capabilities.
o When all the PEs in an ES advertise DF Type 255, they will rely on
the local policy to decide how to proceed with the DF Election.
o For any new capability defined in the future, the
applicability/compatibility of this new capability to the existing
DF Algs must be assessed on a case by case basis.
o Likewise, for any new DF Alg defined in future, its
applicability/compatibility to the existing capabilities must be
assessed on a case by case basis.
3.3 Auto-Derivation of ES-Import Route TargetSection 7.6 of [RFC7432] describes how the value of the ES-Import
Route Target for ESI types 1, 2, and 3 can be auto-derived by using
the high-order six bytes of the nine byte ESI value. The same auto-
derivation procedure can be extended to ESI types 0, 4, and 5 as long
as it is ensured that the auto-derived values for ES-Import RT among
different ES types don't overlap.
4. The Highest Random Weight DF Election Algorithm
The procedure discussed in this section is applicable to the DF
Election in EVPN Services [RFC7432] and EVPN Virtual Private Wire
Services [RFC8214].
Highest Random Weight (HRW) as defined in [HRW1999] is originally
proposed in the context of Internet Caching and proxy Server load
balancing. Given an object name and a set of servers, HRW maps a
request to a server using the object-name (object-id) and server-name
(server-id) rather than the state of the server states. HRW forms a
hash out of the server-id and the object-id and forms an ordered list
of the servers for the particular object-id. The server for which the
hash value is highest, serves as the primary responsible for that
particular object, and the server with the next highest value in that
hash serves as the backup server. HRW always maps a given object name
to the same server within a given cluster; consequently it can be
used at client sites to achieve global consensus on object-server
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mappings. When that server goes down, the backup server becomes the
responsible designate.
Choosing an appropriate hash function that is statistically oblivious
to the key distribution and imparts a good uniform distribution of
the hash output is an important aspect of the algorithm. Fortunately
many such hash functions exist. [HRW1999] provides pseudo-random
functions based on Unix utilities rand and srand and easily
constructed XOR functions that perform considerably well. This
imparts very good properties in the load balancing context. Also each
server independently and unambiguously arrives at the primary server
selection. HRW already finds use in multicast and ECMP [RFC2991],
[RFC2992].
4.1. HRW and Consistent Hashing
HRW is not the only algorithm that addresses the object to server
mapping problem with goals of fair load distribution, redundancy and
fast access. There is another family of algorithms that also
addresses this problem; these fall under the umbrella of the
Consistent Hashing Algorithms [CHASH]. These will not be considered
here.
4.2. HRW Algorithm for EVPN DF Election
This section describes the application of HRW to DF election. Let
DF(v) denote the Designated Forwarder and BDF(v) the Backup
Designated forwarder for the Ethernet Tag v, where v is the VLAN, Si
is the IP address of server i, Es denotes the Ethernet Segment
Identifier and weight is a function of v, Si, and Es.
Note that while the DF election algorithm in [RFC7432] uses PE
address and vlan as inputs, this document uses Ethernet Tag, PE
address and ESI as inputs. This is because if the same set of PEs are
multi-homed to the same set of ESes, then the DF election algorithm
used in [RFC7432] would result in the same PE being elected DF for
the same set of broadcast domains on each ES, which can have adverse
side-effects on both load balancing and redundancy. Including ESI in
the DF election algorithm introduces additional entropy which
significantly reduces the probability of the same PE being elected DF
for the same set of broadcast domains on each ES. Therefore, the ESI
value in the Weight function below SHOULD be set to that of
corresponding ES. The ESI value MAY be set to all 0's in the Weight
function below if the operator chooses so.
In case of a VLAN-Bundle service, v denotes the lowest VLAN similar
to the 'lowest VLAN in bundle' logic of [RFC7432].
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1. DF(v) = Si: Weight(v, Es, Si) >= Weight(v, Es, Sj), for all j. In
case of a tie, choose the PE whose IP address is numerically the
least. Note 0 <= i,j <= Number of PEs in the redundancy group.
2. BDF(v) = Sk: Weight(v, Es, Si) >= Weight(V, Es, Sk) and Weight(v,
Es, Sk) >= Weight(v, Es, Sj). In case of tie choose the PE whose
IP address is numerically the least.
Since the Weight is a Pseudo-random function with domain as the
three-tuple (v, Es, S), it is an efficient deterministic algorithm
that is independent of the Ethernet Tag v sample space distribution.
Choosing a good hash function for the pseudo-random function is an
important consideration for this algorithm to perform better than the
default algorithm. As mentioned previously, such functions are
described in the HRW paper. We take as candidate hash functions two
of the ones that are preferred in [HRW1999].
1. Wrand(v, Es, Si) = (1103515245((1103515245.Si+12345)XOR
D(v,Es))+12345)(mod 2^31) and
2. Wrand2(v, Es, Si) = (1103515245((1103515245.D(v,Es)+12345)XOR
Si)+12345)(mod 2^31)
Here D(v,Es) is the 31-bit digest (CRC-32 and discarding the MSB as
in [HRW1999]) of the 14-byte stream, the Ethernet Tag v (4 bytes)
followed by the Ethernet Segment Identifier (10 bytes). It is
mandated that the 14-byte stream is formed by concatenation of the
Ethernet tag and the Ethernet Segment identifier in network byte
order. The CRC should proceed as if the stream is in network byte
order (big-endian). Si is address of the ith server. The server's IP
address length does not matter as only the low-order 31 bits are
modulo significant. Although both the above hash functions perform
similarly, we select the first hash function (1) of choice, as the
hash function has to be the same in all the PEs participating in the
DF election.
A point to note is that the Weight function takes into consideration
the combination of the Ethernet Tag, Ethernet Segment and the PE IP-
address, and the actual length of the server IP address (whether V4
or V6) is not really relevant. The default algorithm in [RFC7432]
cannot employ both V4 and V6 PE addresses, since [RFC7432] does not
specify how to decide on the ordering (the ordinal list) when both V4
and V6 PEs are present.
HRW solves the disadvantage pointed out in Section 2.2.1 and ensures:
o with very high probability that the task of DF election for the
VLANs configured on an ES is more or less equally distributed among
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the PEs even for the 2 PE case.
o If a PE that is not the DF or the BDF for that VLAN, goes down or
its connection to the ES goes down, it does not result in a DF or
BDF reassignment. This saves computation, especially in the case
when the connection flaps.
o More importantly it avoids the needless disruption case of Section2.2.1 (3), that is inherent in the existing default DF Election.
o In addition to the DF, the algorithm also furnishes the BDF, which
would be the DF if the current DF fails.
5. The Attachment Circuit Influenced DF Election Capability
The procedure discussed in this section is applicable to the DF
Election in EVPN Services [RFC7432] and EVPN Virtual Private Wire
Services [RFC8214].
The AC-DF capability MAY be used with any "DF Alg" algorithm. It MUST
modify the DF Election procedures by removing from consideration any
candidate PE in the ES that cannot forward traffic on the AC that
belongs to the BD. This section is applicable to VLAN-Based and VLAN-
Bundle service interfaces. Section 5.1 describes the procedures for
VLAN-Aware Bundle interfaces.
In particular, when used with the default DF Alg, the AC-DF
capability modifies the Step 3 in the DF Election procedure described
in [RFC7432] Section 8.5, as follows:
3. When the timer expires, each PE builds an ordered "candidate" list
of the IP addresses of all the PE nodes attached to the Ethernet
Segment (including itself), in increasing numeric value. The
candidate list is based on the Originator Router's IP addresses of
the ES routes, but excludes any PE from whom no Ethernet A-D per
ES route has been received, or from whom the route has been
withdrawn. Afterwards, the DF Election algorithm is applied on a
per <ES,VLAN> or <ES,VLAN-bundle>, however, the IP address for a
PE will not be considered candidate for a given <ES,VLAN> or
<ES,VLAN-bundle> until the corresponding Ethernet A-D per EVI
route has been received from that PE. In other words, the ACS on
the ES for a given PE must be UP so that the PE is considered as
candidate for a given BD.
The above paragraph differs from [RFC7432] Section 8.5, Step 3, in
two aspects:
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o Any DF Alg algorithm can be used, and not only the modulus-based
one (which is the default DF Election, or DF Alg 0 in this
document).
o The candidate list is pruned based upon non-receipt of Ethernet A-D
routes: a PE's IP address MUST be removed from the ES candidate
list if its Ethernet A-D per ES route is withdrawn. A PE's IP
address MUST NOT be considered as candidate DF for a <ES,VLAN> or
<ES,VLAN-bundle>, if its Ethernet A-D per EVI route for the
<ES,VLAN> or <ES,VLAN-bundle> respectively, is withdrawn.
The following example illustrates the AC-DF behavior applied to the
Default DF election algorithm, assuming the network in Figure 2:
a) When PE1 and PE2 discover ES12, they advertise an ES route for
ES12 with the associated ES-import extended community and the DF
Election Extended Community indicating AC-DF=1; they start a timer
at the same time. Likewise, PE2 and PE3 advertise an ES route for
ES23 with AC-DF=1 and start a timer.
b) PE1/PE2 advertise an Ethernet A-D per ES route for ES12, and
PE2/PE3 advertise an Ethernet A-D per ES route for ES23.
c) In addition, PE1/PE2/PE3 advertise an Ethernet A-D per EVI route
for AC1, AC2, AC3 and AC4 as soon as the ACs are enabled. Note
that the AC can be associated to a single customer VID (e.g. VLAN-
based service interfaces) or a bundle of customer VIDs (e.g. VLAN-
Bundle service interfaces).
d) When the timer expires, each PE builds an ordered "candidate" list
of the IP addresses of all the PE nodes connected to the Ethernet
Segment (including itself) as explained above in [RFC7432] Step 3.
Any PE from which an Ethernet A-D per ES route has not been
received is pruned from the list.
e) When electing the DF for a given BD, a PE will not be considered
candidate until an Ethernet A-D per EVI route has been received
from that PE. In other words, the ACS on the ES for a given PE
must be UP so that the PE is considered as candidate for a given
BD. For example, PE1 will not consider PE2 as candidate for DF
election for <ES12,VLAN-1> until an Ethernet A-D per EVI route is
received from PE2 for <ES12,VLAN-1>.
f) Once the PEs with ACS = DOWN for a given BD have been removed from
the candidate list, the DF Election can be applied for the
remaining N candidates.
Note that this procedure only modifies the existing EVPN control
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plane by adding and processing the DF Election Extended Community,
and by pruning the candidate list of PEs that take part in the DF
election.
In addition to the events defined in the FSM in Section 3.1, the
following events SHALL modify the candidate PE list and trigger the
DF re-election in a PE for a given <ES,VLAN> or <ES,VLAN-Bundle>. In
the FSM of Figure 3, the events below MUST trigger a transition from
DF_DONE to DF_CALC:
i. Local AC going DOWN/UP.
ii. Reception of a new Ethernet A-D per EVI update/withdraw for the
<ES,VLAN> or <ES,VLAN-Bundle>.
iii. Reception of a new Ethernet A-D per ES update/withdraw for the
ES.
5.1. AC-Influenced DF Election Capability For VLAN-Aware Bundle Services
The procedure described section 5 works for VLAN-based and
VLAN-Bundle service interfaces since, for those service types, a PE
advertises only one Ethernet A-D per EVI route per <ES,VLAN> or
<ES,VLAN-Bundle>. The withdrawal of such route means that the PE
cannot forward traffic on that particular <ES,VLAN> or
<ES,VLAN-Bundle>, therefore the PE can be removed from consideration
for DF.
According to [RFC7432], in VLAN-aware bundle services, the PE
advertises multiple Ethernet A-D per EVI routes per <ES,VLAN-Bundle>
(one route per Ethernet Tag), while the DF Election is still
performed per <ES,VLAN-Bundle>. The withdrawal of an individual route
only indicates the unavailability of a specific AC but not
necessarily all the ACs in the <ES,VLAN-Bundle>.
This document modifies the DF Election for VLAN-Aware Bundle services
in the following way:
o After confirming that all the PEs in the ES advertise the AC-DF
capability, a PE will perform a DF Election per <ES,VLAN>, as
opposed to per <ES,VLAN-Bundle> in [RFC7432]. Now, the withdrawal
of an Ethernet A-D per EVI route for a VLAN will indicate that the
advertising PE's ACS is DOWN and the rest of the PEs in the ES can
remove the PE from consideration for DF in the <ES,VLAN>.
o The PEs will now follow the procedures in section 5.
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For example, assuming three bridge tables in PE1 for the same MAC-VRF
(each one associated to a different Ethernet Tag, e.g. VLAN-1, VLAN-2
and VLAN-3), PE1 will advertise three Ethernet A-D per EVI routes for
ES12. Each of the three routes will indicate the status of each of
the three ACs in ES12. PE1 will be considered as a valid candidate PE
for DF election in <ES12,VLAN-1>, <ES12,VLAN-2>, <ES12,VLAN-3> as
long as its three routes are active. For instance, if PE1 withdraws
the Ethernet A-D per EVI routes for <ES12,VLAN-1>, the PEs in ES12
will not consider PE1 as a suitable DF candidate for <ES12,VLAN-1>.
PE1 will still be considered for <ES12,VLAN-2> and <ES12,VLAN-3>
since its routes are active.
6. Solution Benefits
The solution described in this document provides the following
benefits:
a) Extends the DF Election in [RFC7432] to address the unfair load-
balancing and potential black-holing issues of the default DF
Election algorithm. The solution is applicable to the DF Election
in EVPN Services [RFC7432] and EVPN Virtual Private Wire Services
[RFC8214].
b) It defines a way to signal the DF Election algorithm and
capabilities intended by the advertising PE. This is done by
defining the DF Election Extended Community, which allow signaling
of the capabilities supported by this document as well as any
other future DF Election algorithms and capabilities.
c) The solution is backwards compatible with the procedures defined
in [RFC7432]. If one or more PEs in the ES do not support the new
procedures, they will all follow the [RFC7432] DF Election.
7. Security Considerations
This document addresses some identified issues in the DF Election
procedures described in [RFC7432] by defining a new DF Election
framework. In general, this framework allows the PEs that are part of
the same Ethernet Segment to exchange additional information and
agree on the DF Election Type and Capabilities to be used.
Following the procedures in this document, the operator will minimize
undesired situations such as unfair load-balancing, service
disruption and traffic black-holing. Since those situations may have
been purposely created by a malicious user with access to the
configuration of one PE, this document enhances also the security of
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the network. In addition, the new framework is extensible and allows
for future new security enhancements that are out of the scope of
this document. Finally, since this document extends the procedures in
[RFC7432], the same Security Considerations described in [RFC7432]
are valid for this document.
8. IANA Considerations
IANA is requested to:
o Allocate Sub-Type value 0x06 in the "EVPN Extended Community Sub-
Types" registry defined in [RFC7153] as follows:
SUB-TYPE VALUE NAME Reference
-------------- ------------------------- -------------
0x06 DF Election Extended Community This document
o Set up a registry called "DF Alg" for the DF Alg octet in the
Extended Community. New registrations will be made through the "RFC
Required" procedure defined in [RFC8126]. The following initial
values in that registry are requested:
Alg Name Reference
---- -------------- -------------
0 Default DF Election This document
1 HRW algorithm This document
2-254 Unassigned
255 Reserved for Experimental use This document
o Set up a registry called "DF Election Capabilities" for the two-
octet Bitmap field in the Extended Community. New registrations
will be made through the "RFC Required" procedure defined in
[RFC8126]. The following initial value in that registry is
requested:
Bit Name Reference
---- -------------- -------------
0 Unassigned
1 AC-DF capability This document
2-15 Unassigned
9. References9.1. Normative References
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
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